High pressure cracking furnace and system

ABSTRACT

A high-pressure furnace for cracking hydrocarbons to produce olefin. Long flame burners produce combustion gases to circulate through radiant and convection sections in a furnace under pressure to crack hydrocarbons. Flue gas from the furnace serves to produce high-pressure steam, provide coolant to quench cracked gas, preheat the hydrocarbon-steam feed and aid in driving a turbine-compressor assembly.

United States Patent Woebcke et al. 1 1 Oct. 7, 1975 {541 HIGH PRESSURE CRACKING FURNACE 3,487,121 |2/|%9 Hallec, 260/683 R 3,677,234 7/l972 Dutkicwicz 122/356 X AND SYSTEM [75] inventors: Herman N. Woebcke, Wayland;

Robert J. Gartside, Brighton, both of Mass.

[73] Assignee: Stone & Webster Engineering Corporation, Boston, Mass.

[22] Filed: Nov. 6, 1973 121] App]. No: 413,333

[52] U.S. C1. 23/277 R; 122/240 R; 122/240 B; 122/356; 260/683 R; 431/116; 208/130 [51] 1nt.Cl.-'. C07C 11/02; B011 1/14;C10G 9/20 [58] Field of Search 23/277 R, 284; 208/130; 260/683 R; 122/240 R, 240 B, 356; 431/116 [56} References Cited UNITED STATES PATENTS 2,648,599 8/1953 Throckmorton et al 23/277 R Primary Examiner-lamcs H. Tayman, Jr, Attorney, Agent, or FirmMorgan, Finnegan, Pine, Foley & Lee

[57] ABSTRACT A high-pressure furnace for cracking hydrocarbons to produce olefin. Long flame burners produce combustion gases to circulate through radiant and convection sections in a furnace under pressure to crack hydrocarbons. Flue gas from the furnace serves to produce high-pressure steam, provide coolant to quench cracked gas, preheat the hydrocarbon-steam feed and aid in driving a turbine-compressor assembly.

8 Claims, 5 Drawing Figures U.S. Patent Oct. 7,1975 Sheet 1 of 3 3,910,768

U.S. Patent Oct. 7,1975 Sheet 2 of3 3,910,768

U.S. Patent Oct. 7,1975 Sheet 3 of3 3,910,768

HIGH PRESSURE CRACKING FURNACE AND SYSTEM BACKGROUND OF THE INVENTION l. Field of the Invention This invention relates to furnaces and, in particular, furnaces for cracking hydrocarbons to produce olefins.

2. Description of the Prior Art Presently, the typical thermal cracking furnace is comprised of a radiant heat section and a convection heat section fired frequently by very small capacity radiant heat burners. In some instances the number of burners exceeds one hundred in number for a furnace with a fired duty of 100 MM BTU/hr.

In general, the use of a multiplicity of burners is dietated by present furnace design which requires that the major portion of the heat be transferred to the process fluid by radiation. Illustrative of such furnaces is the furnace which is the subject of U.S. Pat. No. 3,487,121 (Hallee et al, issued Dec. 30, I969).

In the furnace of U.S. Pat. No. 3,487,l2l the radiant Zone has a mean beam length of about to 16 feet which. in that application, achieves rapid heat transfer. However. the radiant heat burners used require gaseous fuels and lack the thermal effieiency of larger flame burners due to the need for excess combustion air. Conversely. large flame burners can fire liquid fuels and even residual liquid fuels and in many cases can be made to operate with nearly stoichiometric air.

The present state of the art does include furnaces fired by long flame burners as illustrated in U.S. Pat. No. 3,677,234 (Dutkiewicz, issued June I8, I972) and Horizontal High Severity Furnace (Woebcke; Ser. No. 362l. filed Jan. I), I970 now U.S. Pat. 3,820,955). In these furnaces employing long flame burners the firing is conducted at atmospheric pressure and the meanbcam length is of the same magnitude or greater than the radiant heat furnaces.

The present furnace art also includes furnaces which are designed to accommodate combustion gas under pressure. In general, these furnaces are designed to permit the combustion gas and the process fluid to operate at essentially the same pressure, thereby relieving the pressure differential on the reactor tubes. The increased pressure on the combustion side of the furnace also facilitates the use of flue gas to drive auxiliary system equipment such as gas turbines. One such furnace, US. Pat. No. 3.688.494 (Mevenkamp, Sept. 5, 1972), is provided with a distinct reaction section and a distinct convection section which are in communication for both the process fluid and flue gas.

SUMMARY OF THE INVENTION The applicants have invented a furnace designed to be operated under pressure and a system for maximiz ing the thermal energy recovered from high pressure tion and through the radiant section. Long flame burners are located in both the radiant section and convection section. The burner in the radiant section is arranged in the center of an internal chimney which has an inner configuration of a converging-diverging section. The combustion gases from the radiant section pass through the interior of the chimney to the top thereof and then to the outside of the chimney where the radiant section process tubes are located. The gases continue their flow over the radiant section tubes through the cross over and ultimately out through the top of the convection section.

The cracked gas quencher is also formed with the furnace and is preferably configured annularly to receive the process tubes which are essentially a continuance of the tubes in the radiant section. Flue gas which has been cooled serves as the coolant for the cracked gases in the quencher.

The flue gases are used to generate high-pressure steam, elevate the temperature of the hydrocarbon steam mixture to be processed and are also used in a turbine compressor assembly. A portion of the flue gases are returned to the furnace through the burners after issuance from the quencher.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional elevational view of the furnace and effluent quencher of the present invention;

FIG. 2 is a sectional view taken through line 2-2 of FIG. 1;

FIG. 3 is a sectional view taken through line 33 of FIG. 1;

FIG. 4 is a sectional view taken through line 4-4 of FIG. 1;

FIG. 5 is a schematic view of the olefin recovery system used with the furnace of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIGS. 1-4 disclose the furnace and effluent quencher of the invention and FIG. 5 discloses the furnace of FIGS. l4 with the associated process equipment.

As best seen in FIG. I, the furnace 2 of the invention is comprised of a convection section 4, a radiant section 6 and a cross-over section 8.

The convection section 4 is comprised of a downstream section 10 and an upstream section 12 with re speet to flue gas flow. The downstream section includes an inlet 14 for the process fluid, a manifold section I6, a plurality of tubes 18 for the process fluid, a hot gas side 20 and an outlet 22 for the flue gas emanating from the hot gas side 20. In the downstream section 10, the process fluid tubes 18 are parallel and the outer shell 24 is cylindrically configured to conform to the contour of the parallel array of tubes I8. In practice, the tubes 18 in the downstream section 10 are arrayed in close proximity at a distance from centers of no more than 6V2 inches and no less than 2% inches. Further. the tubes 18 in the outer concentric circle of the tube array are no more than 6V2 inches and no less than 2% inches from the inner surface 26 of the shell 24 for 2% inch diameter tubes.

The shell 24 of the upstream section 12 of the convection section 4 is flared at an angle of less than 10 from the termination 28 of the downstream section 10 to an intermediate location 30 and is thereafter cylindrically formed. Optionally, the downstream section 10 can be continuously flared to the cross-over section 8. The tubes 18 are arranged at an angle of less than 10 from the termination 28 of the downstream section 10 to the cross-over section 8. Practice teaches that tubes of 2 to 6 inches are particularly suitable for the convection section tubes 18. Alternatively, it may be desirable in some applications to flare an annular passage in the upstream section 12 of the furnace rather than flaring the tubes 18.

The convection section 4 also includes a supplemental burner 32 through which fuel. combustion air and recycled flue gas are fed to the convection section 4. The combustion air is at a pressure of 5 to 15 atm. and preferably atm, the flue gas is at a pressure of 5 to atm. preferably 10 atm, and a temperature of about 1200F at the outlet 22 of convection section 4. The mix of combustion air and flue gas is chosen to provide a pressure of 5 to 15 atm on the combustion side of the furnace 2. Two burners having a capacity of MM BTU/hr, are particularly suitable for use as burners 32 and 44.

The cross-over section 8 is provided to facilitate flue gas flow from the radiation section 6 to the convection section 4 and to accommodate the process tubes 18 for the passage of process fluid from the convection section 4 to the radiation section 6. The tubes 18 are sup ported in the cross-over section 8 with conventional tube support structures.

The radiation section 6 is provided with an essentially cylindrical shell 38, an inner chimney 42. flame burner 44 and an upper refractory roof 46. The chimney 42 has an outer cylindrical surface 48 which converges toward one end and which cooperates with the inner wall 50 of the shell 38 to define an annular passage 52 of varying cross-sectional area. with the largest annular opening formed adjacent the upper refractory roof 46 and the smallest annular opening formed adjacent the burner 44. The inner surface of the chimney 42 is formed of a downstream converging section 54 and an upstream diverging section 56 which combine to form a central flue gas passage 58. The roof 46 is contoured to direct the gases from the central flue gas passage 58 to the annular passage 52. The convergent divergent configuration of chimney 42 promotes gas circulation by causing a draft effect of the flue gas up through the internal passage 58 and down through the annular passage 52.

The process tubes 36 extend upstream from the cross-over section 8 concentrically through the annular passage 52 and through the refractory roof 46 which is contoured to minimize pressure drop in recirculation. The flame burner 44 is centrally disposed in the radiant section 6 at an elevation above the entrance 45 of the chimney 42 but below the beginning of the diverging section 56. Residual fuel such as bunker c heavy fuel oil, combustion air and recycle flue gas are fed to the radiant section 6 through flame burner 44. In practice, the combustion air is at a pressure of 5 to 15 atm and the recycled flue gas is at a pressure of 5 to 15 atm and a temperature of l2U(lF. Again, the mix of components are chosen to provide the hot side of the furnace with a pressure of 5 to 15 atm.

The effluent quencher 62 is mounted immediately upstream of the radiation section 6. The effluent quencher 62 consists of an outer shell 64, and inner wall 66, a cold side inlet 68 and a cold side outlet 70.

The inner surface 72 of the outer shell 64 and the inner wall 66 define the cold side chamber 74 of the quencher 62. Radiant section process tubes 36 extend upwardly through the cold side chamber 74 to a mani' fold 60 which is located on the upstream side of the quencher 62. Recirculated flue gas is used preferably as the coolant in the quencher 62. After passage through the hot side of various preheat equipment and gas booster compressor 140 the flue gas enters the quencher at a temperature of about 500F.

The flue gas leaves the quencher 62 at 1100F to [300F and a portion is ultimately returned to the furnace through burners 44 and 32. The rapid quench exchanger 62 effects countercurrent convection ex change within the annular chamber 74. The small flow area and the high-pressure combustion gas combine to yield convective heat transfer coefficients that are greater than lOO BTU/hr./ft. F. Thus, the quenching operation in the quencher 62 is completed in less than 50 milliseconds, and preferably less than 25 milliseconds.

The composite system of the present invention, best seen in FIG. 5, comprises air compression equipment 75. hydrocarbon feed preheater 78, high pressure steam generation equipment and dilution steam generation equipment 82.

The air compression equipment consists of a turbine 84, a first-stage air compressor 86. a second-stage air compressor 88. a cracked gas compressor and a combustor 92. Fuel. preferably heavy residual fuel oil, is delivered through line 94 to the combustor 92 to form gas to drive the turbine 84. The turbine 84 is arranged to drive compressors 86, 88 and 90. Air from the atmosphere is delivered through line 96 to compressor 86 where it is elevated to pressures in the range of 5 to l5 atm. preferably l0 atm. The compressed air from compressor 86 is delivered through line 98 to both the combustor 92 and compressor 88. The compressed air delivered to the combustor 92 is used to burn the fuel to drive the turbine and the compressed air delivered to compressor 88 may be further compressed to pressures of 5 to 15 atm. preferably 10 atm, and delivered through line 100 to the furnace burners 32 and 44. The gas exhausting from the turbine 84 is delivered to a flue gas exhaust stack 104 by line 102.

The high-pressure steam generation equipment 80 consists essentially of an indirect heat exchange pres sure vessel 106 and a high-pressure steam reservoir I08. Flue gas from the furnace is delivered through line 110 to the hot side of the pressure vessel 106 at temper atures of approximately l200F to prevent a tempcrature pinch.

The hydrocarbon feed preheat equipment 78 consists essentially of a preheater I09 and heat exchanger lll. Hydrocarbon feed which has previously been heated by the exhaust gas from turbine 84 in stack 104 and steam are passed through line 116 to the cold side of preheater 109. Flue gas which is discharged from the steam generation equipment 80 is passed through line 110 to the hot side of the preheater 109. The temperature of the flue gas entering the preheater 109 is in the range of l0O0F and is at a pressure of about 5 to 15 atm, preferably 10 atm. The hydrocarbon-steam feed leaves the preheater 109 at a temperature of about 740F and pressure of 38 to 72 psia and is passed through line 116 to the heat exchanger ll 1. The heat exchanger 111 is an indirect heat exchanger which receives cracked gas from the furnace 2 through line 122 and passes the pyrolysis gas from the furnace through the hot side and hydrocarbon feed and steam through the cold side. The cracked gas enters the heat exchanger III at a temperature of about 1300F and a pressure of about 45 to 25 psia. The hydrocarbon feed is elevated to a temperature of about lOOF and delivered to the furnace 2 by line 116. The hydrocarbon feed and steam enter the furnace in a manifold 16 which is in communication with the convection tubes 18, as best seen in Fl(]. 1.

The dilution steam generation equipment 82 consists of an indirect heat exchanger 124 and an oil heater 126. The indirect heat exchanger 124 is provided with cracked gas through line 128 which leaves the highpressure steam generation vessel ill at a temperature in the range of l()4()F and pressures between 22 and 42 psia. Oil, DOWTHERM, or some other appropriate heat transfer fluid is passed through the cold side of the exchanger 124 in a closed circuit line 130. The heat transfer fluid passes through the hot side of vessel 126 in heat exchange relationship with water to generate dilution steam. The dilution steam is passed through line 132 to the hydrocarbon feed line 116. The cracked gas leaves the heat exchanger 124 at about 450F and at a pressure between 20 and 40 psia. Line 122 delivers the cracked gas to a quench tower 136 where the cracked gas is quenched and thereafter delivered to compressor 90 through line 122 for further processing.

The flue gas from furnace 2 is circulated through the system in a closed circuit. After discharge from the furnace 2 through line 110 the flue gas passes through the hot side of the high pressure steam vessel 106 and then through the hot side of the hydrocarbon feed preheater 109 to a compressor 140 where it is elevated to a pressure of about 5 to 15 atm. preferably atm and the temperature of the flue gas leaving the compressor 140 is about 500F. The 500F flue gas is then delivered to the cold side of the quencher 62. The flue gases are elevated in the quencher 62 to temperatures of approximately i200F and reduced in pressure to about 5 to l5 atm, preferably 10 atm, and thereafter delivered through line 142 to the furnace burners 44 and 32. The flue gas line 142 joins with the fuel line 94 to form common lines 150 which are in direct communication with the burners 44 and 32.

Functionally, the composite system and the furnace 2 operate as follows:

Hydrocarbon feed initially at l00F and 40 to 75 psia is heated in stack 104 to about 600F. Dilution steam at a temperature of about 300F is introduced into the hydrocarbon feed line 116 through which the resultant mixture is delivered at 550F and 40 to 75 psia to the feed preheater 109. The hydrocarbon feed and steam mixture is elevated to a temperature of 740F and a pressure of 38 to 72 psia and delivered to the heat exchanger ll 1 wherein it is further elevated to a temperature of about l00tlF and a pressure of 35 to 70 psia. The hydrocarbon feed and steam mixture is then delivered to the convection section 4 of the furnace 2 at a temperature of l()()()F and pressures of 35 to 70 psia. The resulting gases exit from the radiant section 6 of the furnace 2 at temperatures above U0F and is quenched in quencher 62 to a temperature of about l300F and a pressure of 45 to psiav The flue gas from the furnace 2 exhausts at a temperature of about 1200F and a pressure of 5 to 15 atm,

preferably l0 atm. After passage through the highpressure steam vessel 106, the feed prcheater 109 and the compressor 140, the flue gas is at a temperature of 500F and a pressure of 5 to l5 atm, preferably l0 atm. The flue gas is delivered to the quencher 62 at this pressure and temperature and exhausts therefrom at a temperature of l2tlOF and a pressure of 5 to l5 atm, preferably l0 atm, the temperature and pressure at which the flue gas is ultimately returned to the furnace burners 32 and 44.

In the furnace 2 the hydrocarbon feed-steam mixture passes through the furnace tubes to the quencher in approximately ().25 seconds. The combustion gases in the furnace are maintained at a pressure of i0 atmosphere and pass through the furnace. The temperature gradient of the combustion gas in the furnace ranges from 2l0OF at the burners to l200F at the outlet.

The cracked gas enters the quencher 62 at a temperature of approximately l600F and a pressure of 25 to 45 psia and leaves after 25 milliseconds at 1300F and a pressure of 25 to 45 psia. The composition of the cracked gas leaving the radiant section 6 of the furnace 2 is typical and similar to that disclosed in US. Pat. No. 3,487,12l issued to Hallee.

What is claimed is:

l. A furnace for thermally cracking hydrocarbons. comprising:

a. a convection section for operation with flue gas at a pressure of about 5 to about 15 atmospheres;

b. a radiant section for operation with flue gas at a pressure of about 5 to about 15 atmospheres;

c. a cross-over section in communication with the convection section and the radiant section;

d. process tubes which extend from the inlet at one end of the convection section through the convec tion section, the cross-over section and through the radiant section;

0. at least one long flame burner located in the radi ant section;

f. a chimney shaped member disposed in the radiant section, the outer wall of the chimney shaped member converging toward one end thereof defining with the radiant section inner wall an annular chamber for said radiant section process tubes, said annular chamber having a varying cross-sectional area which decreases toward said long flame burner, and the inner wall of the chimney shaped member defining a central passage for combustion gas, the central passage being shaped to direct the combustion gas therethrough; and

g. means for directing the combustion gas exiting from the central passage of the chimney into said annular chamber of varying cross-sectional area from which it flows into said cross-over section and said convection section.

2. A furnace as in claim 1 further comprising at least one long flame burner in said convection section.

3. A furnace as in claim 1 further comprising an annular quencher mounted on said radiant section, quencher tubes arranged in said annular quencher which are continuations of said process tubes of said radiant section, and means to admit a coolant to said annular quencher for cooling said quencher tubes.

4. A furnace as in claim 1 wherein said process tubes in said convection section are 2.5 inch I.D. tubes, said process tubes in said radiant section are 2.5 inch [.D. tubes.

entry into said annular quencher.

7. A furnace as in claim 1 wherein said process tubes in said convection section are arranged in a parallel relationship to a medial point and thereafter are flared at an angle of less than about 10.

8. A furnace as in claim 1 wherein the outer wall of said chimney shaped member converges toward its upper end to provide an annular chamber of varying cross sectional area, said annular chamber having the greatest cross-sectional area at said upper end. 

1. A FURNACE FOR THERMALLY CRACKING HYDROCARBONS, COMPRISING: A. A CONVECTION SECTION FOR OPERATION WITH FLUE GAS AT A PRESSURE OF ABOUT 5 TO ABOUT 15 ATMOSPHERES, B. A RADIANT SECTION FOR OPERTAION WITH FLUE GAS AT A PRESSURE OF ABOUT 5 TO ABOUT 15 ATMOSPHERES, C. A CROSS-OVER SECTION IN COMMUNICATION WITH THE CONVECTION SECTION AND THE RADIANT SECTION, D. PPROCESS TUBES WHICH EXTEND FROM THE INLET AT ONE END OF THE CONVECTION SECTION THROUGH THE CONVECTION SECTION, THE CROSS-OVER SECTION AND THROUGH THE RADIANT SECTION E. AT LEAST ONE LONG FLAME BURNER LOCATED IN THE RADIANT SECTION, F. A CHIMNEY SHAPED MEMBER DISPOSED IN THE RADIANT SECTION, THE OUTER WALL OF THE CHIMNEY-SHAPED MEMBER CONVERGING TOWARD ONE END THEREOF DEFINING WITH THE RADIANT SECTION INNER WALL AN ANNULAR CHAMBER FOR SAID RADIANT SECTION PROCESS TUBES, SAID ANNULAR CHAMBER HAVING A VARYING CROSS-SECTIONAL AREA WHICH DECREASES TOWARD SAID LONG FLAME BURNER, AND THE INNER WALL OF THE CHIMNEY HAPED MEMBER DEFINING A CENTRAL PASSAGE FOR COMBUSTION GAS, THE CENTRAL PASSAGE BEING SHAPED TO DIRECT THE COMBUSTION GAS THERETHROUGH, AND
 2. A furnace as in claim 1 further comprising at least one long flame burner in said convection section.
 3. A furnace as in claim 1 further comprising an annular quencher mounted on said radiant section, quencher tubes arranged in said annular quencher which are continuations of said process tubes of said radiant section, and means to admit a coolant to said annular quencher for cooling said quencher tubes.
 4. A furnace as in claim 1 wherein said process tubes in said convection section are 2.5 inch I.D. tubes, said process tubes in said radiant section are 2.5 inch I.D. tubes.
 5. A furnace as in claim 3 including means for recirculating flue gas exiting from the furnace as the coolant to said annular quencher.
 6. A furnace as in claim 5 wherein the flue gas coolant in said annular quencher is combustion gas originally at a temperature of approximately 2100*F and a pressure of approximately 10 atm in the furnace and which leaves the furnace convection section at approximately 10 atm and approximately 1200*F, and means for further cooling and recompressing said flue gas to approximately 500*F and 10 atm pressure prior to entry into said annular quencher.
 7. A furnace as in claim 1 wherein said process tubes in said convection section are arranged in a parallel relationship to a medial point and thereafter are flared at an angle of less than about 10*.
 8. A furnace as in claim 1 wherein the outer wall of said chimney shaped member converges toward its upper end to provide an annular chamber of varying cross -sectional area, said annular chamber having the greatest cross-sectional area at said upper end. 